The invention relates generally to a probe for measuring fluid flow and more particularly to a dual transducer probe for measuring fluid flow by employing diverging ultrasound signals according to the Doppler effect principle, especially through blood vessels.
In the field of medical practice, it is often desirable to obtain data regarding blood flow within an individual's blood vessels such as the arteries, veins and capillaries to assist in medical diagnosis, prognosis and treatment. Early microsurgeons appreciated the need for reliable and accurate direct evidence of anastomotic patency such as flap coloration, capillary refill, or peripheral bleeding. Often, surgeons had to rely on a subjective assessment of various types of pulsations postanastomotically.
A dual forceps or "milking" procedure was proposed by J. W. Hayhurst, et al. in "An experimental study of microvascular technique, patency rates and related factors.", Br. J. Plast Surg 28:128-32 (1975). However, this method is disadvantageous due to its traumatic nature and occurrences of proven thrombotic reduction of the lumen by 75-95% in which the anastomoses were classified as fully patent.
An apparatus for measuring the speed of blood flowing through channels by the use of ultrasound according to the Doppler effect principle is described in U.S. Pat. No. 3,766,517 to Fahrbach, the contents of which are incorporated herein by reference. Fahrbach describes a probe having two transmitting/receiving transducers that project a converging signal in which the sending-receiving directions of the transducers form an angle of 90.degree..
Continuous Wave Doppler Ultrasound was described as being useful to assess microvascular anastomotic patency by Van Beek, et al. in "Ultrasound evaluation of microanastomosis", Arch Surg 100:945-949 (1975) Van Beek, et al. describe several velocity profile waveform parameters that they felt would be predictive or ultimate anastomotic patency, but made no substantial attempt to relate these criteria to quantitation of luminal narrowing.
Luminal narrowing was addressed by Freed, et al. in "High frequency pulsed Doppler ultrasound: a new tool for microvascular surgery", J.Microsurg 1:148-153 (1979), the contents of which are incorporated herein by reference. Freed, et al. applied High Frequency Pulse Doppler Ultrasound (HFPDU) to the field of microsurgery. Freed, et al. attempted to quantitate arterial stenoses ranging from 25-99% in vessels having a diameter less than 1 mm using a simple velocity ratio equation: EQU % Area Reductions=100[1-Vp/Vs]
wherein Vp and Vs represent pre-stenotic and stenotic velocities, respectively.
The Freed, et al. technique involves meticulous scanning of a microanastomosis at 0.5 mm intervals with a pencil-type probe. A needle-mounted pencil-type probe 20 shown in FIG. 2, including the piezoelectric-crystal transducer 14' shown in FIG. 1 is employed. Transducer 14' is formed of a 1 mm.sup.2 piezoelectric crystal 11' electrically coupled to a pair of electric lead wires 12a and 12b. A styrofoam acoustic baffle 13' is disposed on an inner surface of crystal 11'.
As shown in FIG. 3, transducer 14 is mounted flush to the end of a twenty gauge needle 21 and is positioned at an angle of 45.degree. to a blood vessel 31 by hand or with a micromanipulator. Transducer 14 is sequentially scanned across the microanastomosis. This technique has been found to be impractical in a clinical setting.
Conventional Doppler effect fluid flow measuring probes have disadvantages. While the probes can be effective for determining whether fluid is flowing through a vessel in some clinical settings, they can be inappropriate for determining quantitative fluid flow values or for pinpointing constricted areas. Thus, conventional probes are not fully satisfactory and have inadequacies due to their shortcomings.
Accordingly, it is desirable to provide an improved fluid flow probe which avoids the shortcomings of the prior art.